The population density and fluctuation of cotton whitefly, Bemisia tzlbaci(Gennadius) adult on squash plants (Cucurbita pepo L) was studied during summer plantations of 2002 and 2003 using four sampling methods (Sticky coloured traps, coloured water pan traps, sweeping net and direct counting on plant leaves) at Qalubiya Governorate. The obtained results could be summarized as follows: the population of whitefly collected was the highest in number by the direct counts recording as total mean number of 732.7 and 2043.3 insects/5 leaves during the first and second season, respectively, followed the insects collected by sticky traps with 788.2&643.2 insect/4 traps and then by water pan trap methods with 322.4&228.6 insect/4 traps during the two seasons, respectively. On the other hand, the sweeping net was the lowest effective sampling method for estimating whitefly population in which the total collected Insects were 84.00 and 60.00 Insects/25 double strokes in average during the two tested seasons, respectively. In relation to the colour traps, the yellow colour plays an important role in trapping B. tabaci adults. The catches In the case of yellow traps were significantly higher than those recorded by using other colures, i.e., blue, green and red traps.